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Abstract

The demand for sustainable materials has led to the development of biopolymers as alternatives to petroleum-based polymers. However, their high flammability limits their use in applications requiring stringent fire safety. Bio-based additives such as lignin and chitosan among others have charring abilities that enhance flame retardancy by forming protective barriers during combustion. Renewable additives such as phytic acid, tannic acid, casein, and hydrophobins also show potential in improving polymer dehydration, which can help create a stable char layer. Phosphorylation and the use of nitrogen-containing compounds and nanomaterials show promise in improving fire resistance. These advancements make biopolymers more suitable for high-risk fire safety applications while maintaining sustainability. While considerable progress has been made in enhancing the flame retardancy of biopolymers, challenges remain in developing highly effective, non-toxic, and fully biodegradable flame retardants that do not compromise the physical properties and processability of the biopolymers.

Keywords

Biobased flame retardant biopolymers flammability

Introduction

Biopolymers, derived from renewable resources, have garnered significant attention due to their sustainability, biodegradability, and biocompatibility. Biopolymers offer a promising alternative to conventional polymers, in applications requiring these properties. However, polymers in general naturally exhibit low temperature endurance and stability. Polymers are prone to ignition and rapid flame spread due to their organic nature. In general, the flammability of polymers is influenced by factors such as thermal stability, degradation pathways, and the presence of flammable volatile compounds.^1,2 Consequently, polymers in general pose a fire hazard in various settings, including households, hospitals, schools, and industrial settings. To mitigate this hazard, flame retardants have been extensively applied as an effective strategy to delay or prevent polymer disintegration and decomposition.^3,4 Enhancing the flame retardancy of biopolymers is crucial for expanding their applications in fields requiring stringent fire safety standards. The aim of this work is to review strategies for flame-retarding biopolymers using environmentally friendly flame retardants/bio-based flame retardants.

Flame retardancy is a concept that involves a material’s ability to reduce flammability and slows the spread of fire.⁵ Flame retardants are commonly used to depress flame propagation chemically and physically in the solid, liquid, or gas phase.^6–8 An effective flame retardant must possess several key characteristics to ensure good flame retardancy. The key characteristics of effective flame retardants include heat absorption.⁹ Heat absorption can occur through endothermic decomposition, where the material absorbs heat as it breaks down. This heat absorption helps to lower the temperature of the material, delaying ignition and slowing the spread of fire. In addition, some flame-retardant release inert products such as water vapor, carbon dioxide, or other non-flammable gases. These gases help to cool the material and dilute the concentration of flammable gases, thus inhibiting combustion. Another possible characteristic is free radical scavenging.¹⁰ Some flame retardants can release reactive products that function as free radical scavengers. These products quench the chemical reactions that occur during combustion by neutralizing free radicals, thereby slowing down or stopping the combustion process. Another possible characteristic of flame retardants is char formation. The char creates an insulating layer on the surface of the burning material.¹¹ The formed char then limits the diffusion of oxygen to the material and decrease the release of combustible gases by trapping volatile degradation products.¹²

The common flame retardants used are based on halogenated compounds.^13,14 Halogenated flame retardants are known for their high effectiveness, particularly those containing chlorine and bromine, but they also come with significant drawbacks. Halogen-containing flame retardants pose significant challenges due to their persistence, bioaccumulation, high toxicity, and adverse environmental impact.^13,15 Due to these drawbacks, halogenated flame retardants have come under increased regulatory scrutiny. Their use is currently under restriction or phased out.^15–19 Numerous other flame-retardant materials, including transition metal (zinc, magnesium, aluminum, antimony, tin) oxides or hydroxides, have been extensively explored for their effective flame retardancy for polymers.^16–19 Phosphorus-based, carbon-based, nitrogen-based epoxy resins, nano-inorganic particles, and so on, flame retardants also offer good fire retardancy, making them a viable alternative to halogen-based flame retardants.²⁰ In addition, organo-phosphorus flame retardants are also widely applied due to their effectiveness and function through both gas-phase and condensed-phase mechanisms.²¹ Insulating char-forming or intumescent flame retardants are also a promising eco-friendly alternative to halogen flame retardants. An intumescent flame-retardant system consists of an acid source, a blowing agent, and a char former. Intumescent technology has long been applied as an advanced system offering flame retardancy to polymeric materials.³ A typical intumescent system for polyolefin-based polymers has been explored.^3,22 The system comprised of ethylene-butyl acrylate-maleic anhydride as the base polymer, ammonium polyphosphate (APP) as both an acid source and a blowing agent, and pentaerythritol (PER) as the carbon source.³ Since then, this technology has attracted substantial interest due to its versatility, effectiveness, and relatively lower environmental impact than traditional flame retardants.¹

A broad spectrum of the aforementioned fire retardants has highlighted their importance in fire safety. Their effectiveness strongly depends on their composition and the mechanisms by which they retard flames.^22,23 The development and use of these materials involve applying them effectively by conducting rigorous testing to ensure performance.^24,25 However, the release of toxic gases during combustion and decomposition, although in relatively small amounts to halogen-containing flame retardants, still pose a significant human and environmental health risks.^13,14,18 Consequently, there is growing interest and demand for flame retardants with reduced or no toxicity and environmental impact. While bio-based materials are increasingly explored for these purposes, it is important to note that bio-based polymers or additives are not inherently non-toxic. The development of relatively safer, low-toxicity flame retardants, including bio-based options, is a key focus to meet the growing consumption of plastics and natural materials.^23,26–29 This trend aims to reduce environmental impacts. Advances in sustainable and eco-friendly flame retardants are shaping the future of this field, promoting the development of more safer and sustainable products.³⁰

Bio-based materials such as lignin, chitosan (CS), phytic acid (PA), proteins, and starch among others are rich in functional groups that include aromatic rings, hydroxyl, phosphorus, and nitrogen groups. These functional groups contribute to intumescent flame retardancy by promoting and contributing to char formation during combustion. Combining bio-based materials with other flame-retardant components, such as APP, organically modified montmorillonite (OMMT), and halloysite nanotubes (HNTs), often results in synergistic effects, enhancing overall flame retardancy.^31–33 Recent studies have also focused on optimizing the dispersion and interaction of bio-based flame retardants within polymer matrices to maximize their efficiency.^34–36 Techniques such as layer-by-layer (LbL) assembly and in situ polymerization have been employed to create nano-structured and well-dispersed flame-retardant systems.³⁰ This review provides a comprehensive overview of diverse types of greener flame-retardant additives, their mechanisms, advantages and disadvantages, methods for developing flame-retardant systems, and recent advancements in bio-based flame-retardant additives for biopolymers.

Brief discussion on traditional flame-retardant systems

Preparation methods

Conventional methods for developing flame-retardant polymers involves using additive flame retardants, which are melt-mixed with polymers.^37–41 Another approach includes incorporating reactive flame retardants directly into the polymer matrix through chemical bonds.⁴² Surface flame retardants are applied as coatings on the polymer product’s surface to enhance fire resistance.^43–48 Each of these methods offers distinct advantages and selected based on the specific requirements of the application, the type of polymer, and the desired level of flame resistance. For instance, inorganic flame retardants, such as aluminum hydroxide (ATH), magnesium hydroxide (MH), zinc borate, APP, nano-clays, and nano-silica, are frequently applied as additive flame retardants though melt-blending.^49–54 Organic flame retardants including halogenated compounds, phosphorus-based compounds, epoxy resins, and nitrogen-based compounds can be applied as additive or reactive flame retardants through melt-blending or by in situ polymerization/grafting, respectively.^55–58 On the other hand, some flame retardants, such as phosphorus-based flame retardants, can be applied as reactive oligomers, additives, and surface coatings. Similarly, intumescent flame retardants, while often used as additive systems blended into the polymer matrix, can also be chemically integrated into the material or applied as surface treatments.^44,45,59,60 These flame retardants provide an effective solution for improving the reaction-to-fire behavior of various materials while offering a potentially less-toxic alternative to halogenated compounds.

Flame-retarding mechanisms

Halogen-based flame retardants function in the gas phase by interfering with the free radical chain reactions involved in combustion.⁶ Tetrabromobisphenol A, hexabromocyclododecane, and polybromodiphenyl ether compounds are commonly used halogenated flame retardants.^61,62 In combustion, such halogenated flame retardants release specific radicals, such as chlorine (Cl•) and bromine (Br•), in the gas phase. The halogen radicals react with the highly reactive combustible H• and OH• radicals to form less-reactive or even inert molecules.^27,63

Red phosphorus and APP are a classic source of phosphorus for flame retardancy.⁶⁴ Phosphorus (P)-based flame retardants primarily act in the solid phase of polymers by contributing to the formation of a protective char layer and are also effective radical scavengers in gas phases.^65,66 They are particularly effective in the condensed phase, especially when applied to oxygen-containing polymers.^6,67 Herein, when exposed to intense heat, compounds like phosphoric acid act as catalysts for polymer dehydration and cross-linking.⁵ In the gas phase, phosphorus-containing flame retardants terminate the fire by releasing phosphorous-containing radicals, such as PO₂•, PO•, and HPO•, to neutralize highly reactive combustible species.⁵ In so doing, the released P- radicals disrupt the flame chemistry, leading to flame inhibition. Phosphorus-based flame retardants are available in various oxidation states, including phosphines (-3), phosphine oxides (-1), elemental phosphorus (0), and phosphates (+5), among others.⁶⁸ Typically, higher oxidation states of phosphorus, like phosphates, tend to act primarily in the condensed phase, promoting char formation.^69,70 In contrast, lower oxidation states, such as those found in phosphines, phosphine oxides, or phosphinates, are more likely to exhibit dual action, enhancing both gas-phase flame inhibition and condensed-phase effects.^71,72 Moreover, phosphorus-based flame retardants are known for their enhanced efficiency when combined with other types of flame retardants, particularly nitrogen-containing flame retardants.⁷² This synergistic use enhances both gas-phase and solid-phase flame-retardant mechanisms. For instance, nitrogen compounds further enhance the dehydration process by catalyzing the polymer phosphorylation step.⁷³ In addition, nitrogen synergists help produce nonvolatile phosphorus acids as by-products, contributing to the overall flame-retardant effect.⁷⁴ Phosphorus-based flame retardants are known for their high effectiveness and lower generation of toxic gases and smoke than halogenated alternatives. But phosphorus-based flame retardants come in various forms, including liquids and solids, and for this reason, some phosphorus compounds including red phosphorus and APP might interact with moisture and produce undesirable volatiles during processing.⁷⁵

Some flame retardants function by cooling or diluting the fuel or by forming a char layer to inhibit combustion.⁷⁶ Endothermic flame retardants produce a cooling effect by decomposing endothermically, absorbing heat, and releasing H₂O, CO₂, or other non-combustible gases.^63,77 Metal-based flame retardants such as metal hydroxides (e.g. aluminum tri-hydroxide, magnesium di-hydroxide), hydromagnesite, and zinc borates decompose endothermically.⁷⁸ Absorption of heat lowers the temperature of the material, making it less likely to ignite or sustain combustion. Metal-based flame retardants are highly effective due to their inherently high temperature resistance and emission of non-toxic gases at elevated temperatures. The challenge with metal-based flame retardants is the large amount of loading required to be effective. This can lead to a significant reduction in the material’s mechanical strength, flexibility, and overall processability.^66,75,79

The incorporation of nanomaterials offers significant advantages as flame retardant.⁸⁰ Nanoparticles, like carbon nanotubes, carbon black, montmorillonite (MMT), silica, and graphene, have been reported to increase the material’s capacity to absorb and store heat (thermal mass), thus promoting the formation of a protective char layer on the surface during combustion.^{33,76,81–84} Herein, as the thermal mass increases, the polymer melt becomes more viscous and less prone to dripping, which also contributes to stable char formation.^85,86 Noteworthy, it is common that melt-dripping from pyrolyzing polymer may dissipate heat and act as a new source of fire. However, in the former case, these nanoparticles may delay volatile gases from fueling the combustion.⁸⁷ The tortuous pathway established by intercalation or exfoliation can act as physical barrier for volatile gases.⁸⁸ Eventually, the rising temperature facilitates the migration of the nanolayers to the surface where it forms a stable char residue.^82,89 On another note, the thermal decomposition of the organomodifier, if present, creates strongly protonic catalytic sites on the inorganic filler surface.⁹⁰ These sites can catalyze the formation of a stable char residue. The char produced functions as a poor heat conductor, creating a layer of insulation that retards heat transfer.⁷⁶ By inhibiting oxygen and hydrogen from the combustion process, the char layer helps to suppress further combustion.^67,91

Intumescent flame retardants, when exposed to fires, undergo chemical reactions resulting in the material foaming, expanding, and charring.³ Intumescent flame-retardant systems typically consist of an acid source, a carbon source, and a blowing agent. The acid source in intumescent systems is typically a polyacid, such as APP or other phosphates. When exposed to elevated temperatures, the acid source decomposes to release inorganic acids. The released acid acts as a dehydrating agent. It removes water from the carbon source, leading to the formation of a charred structure. The carbon source is essential for forming the foamed char. The blowing agent decomposes to release gases such as ammonia (NH₃), steam, and other volatiles. These gases cause the char to expand and form a honeycomb-like structure, enhancing its insulating properties.

Biopolymers and their flammability

Biopolymers have attracted significant attention due to their biological origin and properties such as biodegradability and biocompatibility.⁹² However, their inherent flammability is a significant limitation, especially in applications where fire safety is a concern. In most cases, biopolymers such polylactic acid (PLA), polybutylene succinate (PBS), poly(3-hydroxybutyrate) (PHB), and so on can ignite at temperatures above 270°C, making them susceptible to catching fire during processing or in high-temperature environments. In the UL-94 vertical flame test, biopolymers typically achieve a V-2 rating, demonstrating that they burn with dripping.^93,94 This rating indicates that while a material stops burning within a specified time, it still poses a fire risk due to dripping flaming particles. The tendency of polymers to drip when burning can be both advantageous and disadvantageous, depending on the application. The advantages of dripping include the self-extinguishing behavior, for instance, removal of fuel source and reducing heat transfer. In some cases, the dripping of molten polymer can leave behind a charred residue that helps protect the underlying material from further degradation. However, the disadvantage of dripping poses safety risks in various applications. A disadvantage of dripping includes the creation of secondary fires. In addition, the loss of material due to dripping can compromise the structural integrity of the polymer, leading to failure in applications where mechanical strength is critical. Therefore, enhancing the flame retardancy of these biopolymers is crucial for expanding their application in fields requiring stringent fire safety standards. To address these challenges, various strategies have been employed to enhance their flame retardancy.^95–97

The pursuit of bio-based flame retardants for enhancing the flame retardancy of biopolymers is presently an active area of research. This reflects the growing interest in sustainable and environmentally friendly materials. Bio-based additives and fillers such as lignin, CS, PA, casein, and cellulose are showing promising results in improving the flame retardancy of biopolymers.^98–100 The development of these materials holds significant potential for applications in the packaging, textiles, automotive, and construction industries, where flame-retardant properties are crucial. Furthermore, by combining multiple bio-based additives, researchers aim to develop high-performance biopolymer composites.

Bio-based flame retardants for PLA

PLA is a biodegradable and bioactive thermoplastic polyester derived from renewable resources, such as corn starch or sugarcane.¹⁰¹ PLA is a polyester composed of lactic acid monomers. The polymer can exist in two stereoisomeric forms: poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA). PLA can be processed using conventional plastic-processing techniques, such as injection molding, extrusion, thermoforming, and 3D printing.¹⁰² It is one of the most popular bioplastics used today due to its favorable mechanical properties, biodegradability, and versatility. PLA has attracted significant attention due to its biological origin and properties such as biodegradability and biocompatibility. However, its inherent flammability is a significant limitation, especially in applications where fire safety is a concern.⁹⁸ By incorporating flame retardants, using natural fillers, and employing advanced polymer design techniques, PLA can be tailored to meet the fire safety requirements of various industries. Table 1 summarizes the flame-retardant systems that have been applied to PLA. The table provides quantitative data on the flame performance metrics to illustrate the enhanced effectiveness. The data include key parameters such as time to ignition (TTI), UL-94 rating, limiting oxygen index (LOI), peak heat-release rate (pHRR), and char yield.

Table 1.

Bio-based flame-retardant systems applied on PLA and their effectiveness

Biopolymer matrix	Flame retardant	Method of application	FR loading (Wt.%)	TTI (s)	UL-94	LOI (%)	pHRR (%)	Char residue (%)	Ref.
PLA	Cellulose nanocrystals	Solution based (coating)-Bulk (melt-blending)	1.5	/	V-2	/	/	/	Bajwa et al.¹⁰³
PLA	Cellulose nanocrystals/zinc oxide	Solution based (coating)-Bulk (melt-blending)	1.5/1.5	/	V-2	/	/	/	Bajwa et al.¹⁰³
/	Cellulose	/	/	/	/	/	/	5.27	Dahiya and Rana¹⁰⁴
	Cellulose phenylphosphonate	/	/	/	/	/	/	26.6
	Cellulose 2-dhlorophenyl-phosphonate	/	/	/	/	/	/	33.4
	Cellulose 2-cresylphosphonate	/	/	/	/	/	/	29.5
PLA	/	Solution based (coating)-Bulk (melt-blending)	/	86	/	/	/	0	Costes et al.¹⁰⁵
	Microcrystalline cellulose (MCC)		20	71	/	/	7.70	0
	Phosphorylated MCC		20	46	V-0	/	15.4	10
/	Cellulose fiber	In situ grafting	100	6	/	19	/	4.1	Zhang et al.¹⁰⁶
/	Cellulose/Zn	In situ grafting	95/4.96	59	/	30	66.6	33.6	Zhang et al.¹⁰⁶
PLA	/	Bulk (melt-blending)	0	87	/	/	/	/	Costes et al.¹⁰⁷
	Kraft lignin		20	40	/	/	21	17
	Organosolv lignin		20	27	/	/	∼33	19
	Kraft lignin-PONH4		20	85	V-0	/	∼40	10
	Organosolv lignin-PONH4		20	85	V-0	/	30-33	12
PLA	/	Bulk (melt-blending)	/	/	/	20.0	/	0	Chen et al.¹⁰⁸
	Chitosan/APP	Bulk (melt-blending)	7/0	/	V-2	24.3	/	0.6
	Chitosan/APP		2/5	/	V-0	33.1	17.4	4.1
PLA	/	Layer-by-layer (LbL) assembly	/	48 ± 2	/	22.0	/	0	Battegazzore et al.¹⁰⁹
	Chitosan		9	36 ± 7	/	21.7 ± 0.3	∼50.9	7 ± 1
	Chitosan/sepiolite/APP		3/5/2	33 ± 5	/	25.3 ± 0.3	∼61.1	8 ± 1
PLA	/	Reactive melt-spinning	/	/	/	19.5	/	3.31	Ye et al.¹¹⁰
	Chitosan/PLA/melamine phosphate ester		1	/	/	∼43	/	7.98
	Chitosan/PLA/melamine phosphate ester		>2.5	/	/	∼43	/	0.92
PLA	/	Bulk (melt-blending)	/	37 ± 2	/	20.0	/	0	Zhang et al.¹¹¹
PLA	Casein	Bulk (melt-blending)	20	29 ± 2	V-0	32.2 ± 0.4	∼17.9	4.3	Zhang et al.¹¹¹
PLA	/	Electrostatic self-assembly	/	/	/	20.0	/	/	Zhong et al.¹¹²
PLA	Phytic acid (PA)/nicotinamide	Electrostatic self-assembly	2	/	V-0	29	/	/	Zhong et al.¹¹²
PLA	/	Reactive—bulk (melt-blending)	/	64	V-2	20.0	/	0.56	Liao et al.¹¹³
PLA	Tannic acid/APP	Reactive—bulk (melt-blending)	5	52	V-0	27.0 ± 0.1	∼8.6	7.78	Liao et al.¹¹³

Cellulose

Cellulose is the most abundant organic polymer on Earth, found primarily in the cell walls of plants.¹¹⁴ It is a key structural component of many natural fibers, including cotton, hemp, flax, and wood. Cellulose is a linear polysaccharide composed of D-glucose units linked by β-1, 4-glycosidic bonds.^115,116 The glucose units are arranged in a linear chain, allowing the formation of hydrogen bonds between hydroxyl groups on adjacent chains. This results in a highly crystalline structure, which contributes to cellulose’s strength and insolubility in water.^114,116 The hydrogen bonding leads to the formation of microfibrils, which are bundles of cellulose chains that provide structural integrity to plant cell walls.¹¹⁷

Cellulose derivatization involves substituting some of the hydroxyl groups on the cellulose chain with other functional groups.¹¹⁶ Common derivatives include cellulose acetate, cellulose nitrate, and carboxymethyl cellulose (CMC). Cellulose derivatization involves modifying cellulose to alter its properties, making it more suitable for plastic production. This process improves solubility and thermoplasticity. Plasticization involves adding plasticizers to cellulose derivatives to improve their flexibility and processability.¹¹⁸ Plasticizers are mixed with cellulose derivatives to reduce intermolecular forces, allowing the material to flow and be molded when heated. This process turns rigid cellulose derivatives into flexible, workable plastics. These cellulose-based materials offer a sustainable alternative to conventional plastics, aligning with growing environmental concerns and the demand for biodegradable products.

Cellulose has a low fire resistance, indicated by a limiting oxygen index of 19%.¹¹⁹ However, its ability to produce thermally stable charred residues when exposed to fire is an advantage for its use in flame-retardant systems.¹²⁰ During combustion, particularly under conditions of limited oxygen, cellulose thermally degrades into char, gases, and volatile compounds. The phenomenon of char formation during the burning of wood is well documented.¹²⁰ Due to this property, cellulose is considered a suitable flame-retardant filler for biopolymer-based applications.¹²¹ Table 5 summarizes the flame-retardant systems incorporating cellulose. The table provides quantitative data on the flame performance metrics to illustrate the enhanced effectiveness. The data include key parameters such as TTI, UL-94 rating, LOI, PHRR, and char yield.

Various modifications have been proposed to enhance the flame-retardant properties of cellulose-containing flame retardants. Modifications of cellulose often involve chemical treatments via surface treatment or polymer/cellulose grafting.¹²² Phosphorylation of cellulose is another well-established method to enhance flame retardancy of cellulose-based flame retardants.^123,124 Phosphorylation involves introducing phosphate groups into the cellulose structure.¹²⁵ Phosphorylation can enhance the flame retardancy of cellulose materials by promoting char formation, reducing flammability, and increasing thermal stability.¹²⁶ Research by Dahiya and Rana highlighted that phosphorus-containing cellulose significantly improves char production.¹⁰⁴ Costes and co-workers¹⁰⁵ explored compositions involving phosphorus-containing cellulose for sustainable flame-retarding PLA. In the study, phosphorus groups were either chemically grafted onto cellulose fibers, which were then incorporated into the PLA matrix, or PLA was blended with cellulose phosphorylated using a bio-based phosphorus agent, such as aluminum phytate. Both methods resulted in enhanced charring effects in cellulose. This modification aligns with the growing demand for sustainable and non-toxic flame-retardant solutions, making phosphorylated cellulose a valuable material for a wide range of applications.

In their study, Feng and co-workers⁹⁸ introduced an innovative method by chemically grafting a phosphorus–nitrogen-based flame retardant onto the surface of cellulose nanofibers (CNF) in situ for use in PLA. Their study revealed that incorporating 10 wt.% of the phosphorus–nitrogen flame retardant on CNFs into PLA enabled the material to achieve a V-0 flame resistance rating in vertical burning tests. In addition, it significantly reduced the pHRR in cone calorimetry tests, indicating a marked decrease in flammability. The tensile strength of PLA also increased by about 24%, reaching approximately 72 MPa. This work provided a novel approach to creating advanced green polymeric materials by combining exceptional flame retardancy with mechanical enhancement into a single hierarchical nano-structured additive system.

Combining cellulose with nanoparticles can be utilized to enhance the flame retardancy.^127,128 Recent studies have demonstrated that treating cellulosic materials with nano-sized zinc oxide (ZnO) can significantly enhance the flame retardancy of base polymers.¹⁰³ This approach offers a promising route to improve the fire resistance of cellulose-based flame retardants. Kabir and co-workers¹²¹ proposed a mechanism illustrating the fire retardancy of ZnO-modified cellulose nanocrystal (CNC)-based polymers, shown in Figure 1. ZnO is known for its thermal stability and ability to catalyze char formation, while CNCs are highly crystalline and have a high surface area, making them effective as reinforcing agents and promoting char formation.^103,129 The mechanism illustrates how the combination of these materials enhances the suppression of volatile emissions and char formation providing both solid- and gas-phase fire-retardant actions.¹²¹

Figure 1.

Mechanisms of ZnO-coated cellulose nanocrystals (CNC)/polymer char microstructure.¹²¹

Lignin

Lignin has been explored as a sustainable flame-retardant additive for PLA and other biopolymers.¹³⁰ Lignin is the second-most-abundant biopolymer after cellulose.^131,132 Lignin is rich in phenolic units and has a highly cross-linked structure.¹⁰⁷ Its potential as a flame retardant is attributed to its high char-forming capability.¹³³ Lignin serves as a carbon source in the condensed phase, promoting char formation, and quenches free radicals in the gas phase via phenolic groups; however, these mechanisms depend on lignin’s origin, structure, and concentration.^134,135 Costes and co-workers¹⁰⁷ studied the fire properties and thermal behavior of PLA/lignin composites containing 20 wt.% of both unmodified and modified lignin. The results were evaluated using cone calorimetry, UL-94 tests, and thermogravimetric analysis (TGA). Indeed, the TGA results demonstrated that incorporating untreated lignin into PLA resulted in a flame-retardant effect primarily due to char formation. However, this also caused a significant reduction in the thermal stability of PLA and a notable decrease in its TTI. This means that while untreated lignin helped in forming a protective char layer, it also made PLA more susceptible to thermal degradation and quicker to ignite. In contrast, lignin that was chemically treated with phosphorus (P) and nitrogen (N) showed more favorable results. The treated lignin was effective in limiting the thermal degradation of PLA during both melt-processing and TGA experiments. In addition, P-N-containing lignin significantly improved the flame-retardant properties of the PLA composites, achieving a V-0 classification in the UL-94 test. The results indicated that the PLA composite with P-N-modified-lignin not only resists ignition but also self-extinguishes quickly when ignited.

Yang and co-workers also attained desirable flame retardancy for PLA/phosphorus-nitrogen modified lignin composites, achieving a UL-94 V-0 rating and meeting industrial standards.¹³⁶ In their study, the phosphorus-nitrogen-modified lignin was embedded into PLA through condensation reactions. A study by Zhang and co-workers¹³⁷ has also demonstrated the successful use of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and hexamethylene diisocyanate–modified lignin flame retardants in synthetic polymers. The effectiveness of the lignin-based flame retardant was attributed to both its ability to form an insulating char and the presence of phosphorus-nitrogen groups as modifiers.¹³⁸ In addition, because reactive flame retardants become a part of the polymer’s molecular structure, they are anticipated to have less of a negative impact on the polymer’s mechanical properties. For instance, Zhang and co-workers¹³⁹ synthesized a reactive flame retardant using bio-derived multifunctional additives, including DOPO, diphenolic acid (DPA), and phenylphosphonic dichloride (PPDC), compound abbreviated as PPD. The effectiveness of the synthesized flame retardant was evaluated on the flame retardancy and mechanical properties of PLA. It was found that adding 6 wt.% PPD to PLA resulted in an LOI value of 27.1% and a pHRR of 388 kW/m², compared to the PLA’s original values of 20.5% LOI and 417 kW/m² pHRR. In addition, the composite achieved a UL-94 V-0 rating, indicating excellent flame retardancy, and demonstrated a significant increase in char residue yield. The PLA/6% PPD composite also showed a 31% increase in impact toughness and a significant improvement in tensile strength compared to the bulk PLA. The results were attributed to fine and uniform dispersion of PPD within the resin matrix, along with strong interfacial interactions. Similar observations have been reported for APP grafted onto lignin and PA grafted onto bamboo charcoal through the in situ solid-phase polycondensation.^140,141 Reactive flame retardants not only ensures long-term effectiveness but also addresses environmental concerns associated with the leaching of flame retardants from the host polymer matrices.¹⁴² This method is effective in mitigating potential environmental impacts and improving sustainability of the flame-retardant polymers.

The study by Zhang and co-workers¹⁴³ underlines the effectiveness of combining lignin, microencapsulated ammonium polyphosphate (MAPP), and OMMTs in enhancing the flame retardancy of PLA. Microencapsulation of flame retardants has proven to be an effective strategy in conjunction with melt-mixing.^144–146 Microencapsulation of the flame retardants allows for improved distribution and stability of the flame-retardant materials within the polymer matrix.^41,147,148 The study showed the combination of lignin, MAPP, and organically modified MMT provided PLA with superior flame retardancy and thermal properties compared to systems without the combined components. This combination not only improved the formation of a high-quality, durable char layer but also offered a sustainable approach to developing flame-retardant materials. The synergistic effect of these components improved the overall fire resistance of the PLA.

Chitosan

CS has also been investigated as an effective charring agent because of its rich carbon, nitrogen, and hydroxyl functionalities.^149,150 In flame retardancy applications, CS acts primarily in the condensed phase, to form a more stable carbonous char layer.^151,152 Battegazzore and co-workers¹⁰⁹ used LbL assembly to coat CS to simultaneously improve flame-retardant and mechanical properties of PLA biocomposites. The study demonstrated reduced LOI values, substantial reduction in pHRR, and maximum average rate of heat emission during cone calorimetry while maintaining the mechanical strength for the composites. In another study, Chen and co-workers¹⁰⁸ incorporated CS alone and a combination of CS/APP into PLA by melt-compounding. While the incorporation of CS alone into PLA did not yield satisfactory flame-retardant performance, the CS/APP combination significantly enhanced the flame-retardant properties of the composite. This was evidenced by improved time of ignition, UL-94 ratings, higher limiting oxygen indices, and reduced heat-release rates and total heat released in cone calorimeter tests. These findings underscore the critical role of combining CS with phosphates in APP to achieve effective flame retardancy. Reactive flame-retardant techniques have also been used to enhance CS-based flame retardants. In a recent study, PLA and a melamine phosphate ester were grafted onto a CS framework to create a flame retardant.¹¹⁰ Adding a small amount (<1%) of the CS-based flame retardant to the PLA effectively enhanced both flame retardancy and mechanical properties. However, it was also noted that an excessive amount (>1%) of CS-based flame retardant led to a decline in mechanical performance. This highlights the importance of optimizing the concentration of flame retardant to balance flame resistance and mechanical integrity. Furthermore, a clear synergistic effect between CS and APP was observed, meaning their combined effect on improving flame retardancy was greater than their individual effects. A study by Zhang et al.¹⁵³ demonstrated the effective performance of CS and alginate, which were self-assembled with APP in an aqueous solution. At 10 wt.% loading of APP/CS/AA into PLA, the LOI value increased to 30.6%, UL-94 V-0 rating was achieved, and a 23.1% reduction in the pHRR was observed. Thermogravimetric infrared (TG-IR) analysis supported that the flame-retardant action occurs mainly through a condensed-phase mechanism, evidenced by reduced peak intensities of volatile degradation products. These results indicated a suppressed release of flammable gases. This was attributed to both CS and alginate acting as carbon agents. The mechanical testing showed an impact strength of 4.3 kJ/m² and elongation at break of 9.8%, with a trade-off in tensile strength, decreasing to 46.4 MPa from 56.7 MPa for neat PLA. These results suggested effective flame retardancy with a minor compromise in tensile strength. Indeed, this work highlights the potential of using CS/alginate combination, although further improvements in mechanical properties may be necessary for applications requiring high structural integrity. In addition, the study provides limited insight into the detailed molecular interactions between the APP, CS, and alginate. Finally, alginate systems tend to be more hydrophilic than CS, raising concerns about the composite’s stability and flame retardancy under humid conditions.^154,155 Addressing these limitations could provide a more balanced flame-retardant system with improved practical applicability.

Casein and other proteins

Casein is a protein found in milk, and it consists of phosphorus-nitrogen groups.¹⁵⁶ Due to the known P-N synergistic effects in flame-retardant applications, casein has been explored as a flame-retardant filler.¹⁵⁷ The casein flame-retardance mechanism involves the release of non-flammable NH₃, H₂O to act in the gas phase, and subsequent phosphorus-containing acids such as phosphoric acids that dehydrate the polymer, leading to the formation of a stable char.^158,159 Zhang and Jin explored casein as flame retardant for PLA, and the results showed that incorporating casein into PLA significantly enhanced PLA’s flame retardancy evidenced by higher LOIs.¹¹¹ Furthermore, Figure 2 displays digital photographs of (a) PLA and (b) PLA/20 wt.% casein samples after the UL-94 tests at different times. The combustion of pure PLA showed significant burning flame with melt-dripping and no formation of a protective char layer. On the contrary, the digital photographs of the combustion of the PLA/20 wt.% casein provided clear visual evidence of the enhanced flame retardancy through char formation, however, also with melt-dripping. Although the addition of casein reduced tensile strength of the composite, the dripping occurred due to decreased T_g as heating resumed. Conversely, the dripping helped extinguish the flame, as it did not ignite the cotton. Thus, the PLA-casein composited achieved a V-0 rating, demonstrating effective flame retardancy. The study also predicted the proposed casein dual-action flame-retardant mechanism in the gas and condensed phase. Supporting thermogravimetric analysis coupled with fourier transfom infrared spectroscopy (TGA-FTIR) analyses showed intense peaks (965 cm^–1 and 930 cm^–1 and 3332 cm^–1), corresponding to the NH³ gas at 268.8°C and 380.8°C, respectively, and two major peaks at 1596 cm^–1 and 1256 cm^–1 at 551°C corresponding to the P–C bond and the P–O–P groups, respectively. While the results are promising, the reduced TTI in the PLA/casein composite suggests reduced thermal stability. This shortcoming may limit casein’s effectiveness in certain high-heat applications and impact the durability of the flame retardancy over time. This highlights the need for further modification of the casein to improve fire performance without compromising material stability.

Figure 2.

Digital photographs of (a) PLA and (b) PLA/20 wt.% casein samples after the UL-94 tests at different times.¹¹¹

Dong and co-workers also made significant progress in developing a bio-based flame retardant using soybean protein.¹⁶⁰ Their study investigated a dimethyl phosphate–modified soybean protein abbreviated as SPDP. Figure 3 illustrates both the synthesis process of SPDP and its flame-retardant mechanism in PLA. The study also predicted that SPDP provides a dual-action approach to fire prevention. The proposed mechanisms occurred through char formation and gas-phase dilution. Upon combustion, SPDP releases phosphorus-containing acids, such as phosphonic acid, polyphosphate, or metaphosphate, which contribute to dehydration and char formation. Subsequently, the protein releases NH₃, a non-flammable gas. The released NH₃ helps dilute the concentration of flammable gases around the PLA. While Dong and co-workers made great progress, some aspects could be further explored to solidify their findings. For example, the study suggested the mechanism via NH₃ gas-phase dilution, however, lacks comprehensive kinetic analysis to support these conclusions. Indeed, the release of NH³ is proposed as beneficial, yet quantifying the actual concentration and the impact of NH³ could enhance the understanding of its role in diluting flammable gases.

Figure 3.

Synthesis route of soybean phosphorus-nitrogen-containing flame retardant (SPDP) and the flame retardancy mechanism.¹⁶⁰

Phytic acid

PA is considered an eco-friendly, bio-based alternative in flame retardancy, due to its phosphorus and hydroxyl groups.¹⁰⁰ PA’s role in flame-retardant system involves the release of phosphoric acid or other phosphorus compounds, thereby interrupting the free radical combustion reaction while promoting char formation.^161,162 Xu and co-workers¹⁶³ synthesized a fire retardant combining PA and arginine (AR) in aqueous phase. Upon the addition of 1 wt.% PA-AR, the LOI value of PLA reached 26.8% and UL-94 V-0 rating. In a more recent study by Zhong and co-workers,¹¹² the combination of PA and nicotinamide as flame retardants for PLA showed improved flame retardancy for PLA. The addition of just 2 wt.% of PA and nicotinamide increased the LOI of PLA to 29.0% and gave a UL-94 V-0 rating. Xiong and co-workers⁹⁹ explored a greener strategy for synthesizing PA/CS bio-based flame retardants for PLA.¹⁶⁴ The study employed water as the solvent for the LbL method. TG-IR analysis demonstrated that this system effectively suppresses the release of volatile components during degradation, as evidenced by weaker absorption intensities. The barrier effect of the char formed suggested the condensed-phase mechanism was predominantly responsible for reducing combustible gas emissions. On the other hand, although elongation at break increased from 8.1% to 10.4%, this might not fully compensate for the loss in tensile strength, 56.7–45.3 MPa, especially in scenarios where high strength and flexibility are both crucial. Finally, the lack of comparison to similar water-based LbL assembly methods leaves a gap in understanding how this approach performs relative to other environmentally friendly flame retardants.

Tannic acid

Tannic acid (TA) is a natural catechol derivative, known to exhibit strong antioxidant properties due to its numerous phenolic hydroxyl groups.¹⁶⁵ In the flame retardancy mechanism, TA helps reduce the production of free radicals through its rich phenolic structure while releasing active phenolic radicals.^166,167 TA is also said to promote the formation of a stable char layer during combustion.¹⁶⁸ However, TA is less effective on its own as an intumescent flame retardant.¹⁶⁹ So, TA is typically combined with other compounds to enhance its flame retardancy. Several studies have explored various combinations of TA with other substances to improve the flame retardancy of PLA.^113,169,170 Qiu and co-workers developed a bio-based flame-retardant additive synthesized from TA, CS, and PA.¹⁶⁸ This flame retardant was created using a free radical grafting process followed by electrostatic adsorption. When a low addition of 3% PA/TA -CS was introduced into PLA via melt-blending, the resulting composite exhibited significant flame retardancy, achieving an LOI value of 26.9% and a UL94 V-0 rating. Laoutid and co-workers¹⁶⁹ prepared three combinations of TA with OMMT, metallic phytate salt, and by chemical modification of TA through grafting phosphoric acid groups. Another study by Laoutid and co-workers¹⁷⁰ explored the antioxidative properties of a combination of TA and PA with polyethyleneimine (PEI), serving as a binding agent. Liao and co-workers¹¹³ developed a novel integrated intumescent flame-retardant system that contained APP as the core material, with PEI serving as a bridging agent to connect APP and TA. Overall, these studies found a significant improvement of the flame retardancy of PLA biocomposites, as evidenced by higher LOI values, successful UL-94 V-0 classification, and a reduced pHRR. These improvements highlight the effectiveness of using TA as flame-retardant additives in PLA-based materials. These combinations were aimed at leveraging the synergistic effects of these materials to improve the flame retardancy of PLA.

Bio-based flame retardants for PBS

PBS is a biodegradable aliphatic polyester gaining attention as a promising alternative to traditional petrochemical-based plastics. This is due to its biodegradability and renewable sourcing options. PBS is composed of repeating units of butylene succinate, forming a linear polyester chain.⁹⁸ PBS can be synthesized from petrochemical-based succinic acid and 1,4-butanediol and from biomass fermentation processes. PBS can be processed using conventional plastic processing techniques such as injection molding, extrusion, and thermoforming.¹⁷¹ It can be easily blended with other polymers or additives to modify its properties and improve performance.¹⁷² It is reasonable to predict that with the increasing applications of biopolymers, flame retardancy of PBS will play an essential role in its demand. Table 2 summarizes the flame-retardant systems that have been applied to PBS. The table provides quantitative data on the flame performance metrics to illustrate the enhanced effectiveness. The data include key parameters such as TTI, UL-94 rating, LOI, pHRR, and char yield.

Table 2.

Bio-based flame-retardant systems applied on PBS and their effectiveness

Biopolymers matrix	Flame retardant	Method of application	FR loading (Wt.%)	TTI (s)	UL-94	LOI (s)	pHRR (%)	Char residue (%)	Ref.
PBS	/	Bulk (melt-blending)	30	96	/	/	/	1.2 ± 0.3	Wang et al.¹⁷³
	Lignin/cellulose			42 ± 3	/	/	43.2	7.3 ± 0.9	Wang et al.¹⁷³
PBS	/	Grafting—Bulk (melt-blending)	/	43 ± 1	/	/	/	0.1 ± 0.1	Chen et al.¹⁷⁴
	Phosphorus-nitrogen lignin		30	31 ± 1	/	/	/	9.6 ± 0.1	Chen et al.¹⁷⁴
PBS	/	Additive melt-blendingGrafting	/	72	/	/	/	5.7	Ferry et al.¹⁷⁵
	Alkali lignin		20	42	/	/	49.0	17.0
	Phosphorus-alkali lignin		20	40	/	/	52.0	14.8
PBS	/	Reactive—bulk (melt-blending)	/	/	/	21.28	/	4.9	Kahkesh and Rafizadeh¹⁷⁶
	Boehmite nanoparticles		2			22.85	/	10.3	Kahkesh and Rafizadeh¹⁷⁶
PBS	/	Bulk (melt-blending)	/	63.3 ± 1.5	/	/	/	0.12	Mokhena et al.¹⁷⁷
	Expanded graphite/clay nanoparticles		6/3	50.0 ± 5.3	/	/	40.5	8.55	Mokhena et al.¹⁷⁷
PBS	/	Bulk (melt-blending)	/	48 ± 1	/	21.0	/	0.1 ± 0.1	Chen et al.¹⁷⁸
	Nano-sized carbon black		10	39 ± 3	/	28.5	61.6	11.7 ± 0.2	Chen et al.¹⁷⁸
PBS	/	Bulk (melt-blending)	/	50	/	24.5	/	3	Wang et al.³¹
	Halloysite nanotubes/(APP/melamine/pentaerythritol)		1.5/28.5	40	V-0	58.2	81.4	32	Wang et al.³¹
PBS	/	Bulk (melt-blending)	25	46	/	20.8	/	0.1	Kong et al.¹⁷⁹
	Phytic acid arginine salt		23.5/1.5	22	V-2	26.4	41.4	13.4
	Phytic acid arginine salt/nano-montmorillonite			25	V-0	31.4	74.1	15.3

Lignin

Lignin has been explored as a sustainable flame-retardant additive for PBS.¹⁷³ Recently, Wang and co-workers¹⁷³ observed a positive effect on flame retardancy using chemically untreated cellulose- and lignin-based fillers, eucommia residue (ER), which contained 18 wt.% cellulose, 13 wt.% hemicellulose, and 35 wt.% lignin. The ER was explored as a flame retardant for PBS in PBS/ER composites, with ER loadings ranging from 5% to 30%. The proposed reinforcing mechanism involved the condensed aromatic structures of lignin, which led to increased char formation, exerting a barrier effect. In addition, it was suggested that the cellulose- and lignin-based fillers effectively suppressed the smoke release behavior of PBS. This suppression was attributed to the increased carbon residue content, which reduced the release of aromatic or other small unstable carbon particles produced during combustion.

Ferry et al.¹⁷⁵ and Chen et al.¹⁷⁴ prepared phosphorus-nitrogen-modified lignin/PBS composites by melt-mixing and grafting, respectively. In their studies, lignin enhanced the char residue, contributing to the material’s flame-retardant properties. Moreover, the phosphorylation of lignin made it a particularly effective bio-based flame-retardant component.^138,174,175 Figure 4 shows the cone calorimetry results by Chen and co-workers¹⁷⁴ after adding 10%, 20%, and 30% by weight of a phosphorus–nitrogen-containing lignin (CP-lignin) fire retardant into PBS. The PBS/lignin composite had a significantly lower pHRR and total heat release (THR) than neat PBS. Likewise, the composites produced more char, as observed in Figure 4. The increase in the amount of char enhanced the material’s ability to delay flammable gas transfer and resist burning, thus improving its flame-retardant properties.

Figure 4.

(a) HRR and (b) THR versus time curves, and cone calorimeter digital photos of (c) PBS, (d) PBS/10 wt.%-CP-lignin, (e) PBS/20 wt.%-CP-lignin, and (f) PBS/30 wt. %-CP-lignin.¹⁷⁴

Nanoparticles

It is well known that mineral/inorganic nanoparticles enhance the barrier properties and heat resistance of polymers. Boehmite (Bhm) nanoparticles, for instance, significantly improved the flame retardancy of PBS at 2 wt.% filler content.¹⁷⁶ Other nano-flame retardants, that is, carbon black, nano-clay, HNTs, carbon nanotube, have also been used to improve the pHRR and limited oxygen index value and to inhibit/reduce melt-dripping of PBS.^{31,177,178,180,181} The mechanism of action of these nano-flame retardants involved hindering/delaying the diffusion of oxygen and heat into the interior substrate, which not only delayed melt-dripping but also contributed to the formation of a more stable charred layer.^182,183

Some claims have been made regarding the synergistic effect between phosphorus-based and nanoparticle flame retardants.¹⁸⁴ The study by Kong and co-workers¹⁷⁹ highlights the combined effects of a bio-based phytic acid arginine salt (PaArg) and hydroxylated MMT (abbreviated as DK2) for PBS. Figure 5 serves as a visual summary of the proposed mechanism of action for the synergistic effect between the bio-based PaArg/DK2 for PBS. The mechanisms and synergistic effects involved the phosphorus, from PA, catalyzing the charring process and acting in the gas phase (as HPO• and PO•) to quench free radicals. Upon the addition of MMT, the clay layers reinforced the char structure by increasing the viscosity of the polymer melt, reducing the gas-release rate and forming a filler-packed foam-like structure. The study highlighted an improvement in flame retardancy including LOI enhanced to 31.4%; notable reductions in pHRR, THR, and smoke production; and a V-0 rating. However, the mechanical properties of the composite require closer examination. The measured tensile strength, bending strength, and impact strength of the composite have all been compromised compared to pure PBS. Thus, while the study contributes to the development of bio-based flame retardants, further research could strengthen the evidence for the claimed synergy. In addition, as the MMT is proposed to reinforce the char structure and increase melt viscosity, further data, such as rheological analysis, could better substantiate this claim.

Figure 5.

Mechanism and synergistic effects of PBS/PaArg/DK2.¹⁷⁹

Bio-based flame retardants for PHB

PHB is a biodegradable thermoplastic polymer produced by various microorganisms as an energy storage compound. PHB is a polyester composed of 3-hydroxybutyrate monomers.¹⁰¹ The monomers are linked by ester bonds, forming a linear polymer chain.¹⁸⁵ It is part of the polyhydroxyalkanoates (PHA) family and is of great interest for sustainable and environmentally friendly applications. PHB can be processed using conventional plastic-processing techniques, such as injection molding, extrusion, and thermoforming. However, its narrow processing window requires precise temperature control to prevent thermal degradation.¹⁷¹ In addition, PHB is inherently flammable, which significantly limits its applications, especially where fire safety is a concern.¹⁸⁶ By incorporating flame retardants, using natural fillers, and employing advanced polymer design techniques, PHB can be tailored to meet the fire safety requirements of various industries. Table 3 summarizes the flame-retardant systems that have been applied to PHA. The table provides quantitative data on the flame performance metrics to illustrate the enhanced effectiveness. The data include key parameters such as TTI, UL-94 rating, LOI, pHRR, and char yield.

Table 3.

Flame-retardant systems applied on PHAs and their effectiveness

Biopolymers matrix	Flame retardant	Method of application	FR loading (Wt.%)	TTI (s)	UL-94	LOI (%)	pHRR (%)	Char residue (%)	Ref.
PHBV/PBAT	/	Bulk (melt-blending)	/	22	/	/	/	/	Gallo et al.¹⁸⁷ and Russo et al.¹⁸⁸
	Kenaf fibers		30	25	/	/	50	9.9	Gallo et al.¹⁸⁷ and Russo et al.¹⁸⁸
PHB	/	Bulk (melt-blending)	/	45 ± 3	/	23.9	/	0.0 ± 0.1	Battegazzore et al.¹⁸⁹
	Hemp hurd (HHC) fibers		30	30 ± 1	/	20.1	33	6.5 ± 0.1
	Alfalfa (AA) fibers		30	29 ± 1	/	20.8	32	7.8 ± 0.1
PHA	PHA	In situ intercalation—bulk (melt-blending)	/	15	V-0	19.8	/	7.3	Xu et al.⁹⁴
	Hyperbranched macromolecules/bentonite		15	10		29.6	/	13.8	Xu et al.⁹⁴
P(3HB-co-4HB)	/	In situ intercalation—bulk (melt-blending)	/	55 ± 1	/	/	/	4.3 ± 0.4	Wu et al.¹⁹⁰
	Rice husk ash		5	48 ± 2	/	/	13.8	11.7 ± 0.5
	Alkali lignin/rice husk ash		10/5	35 ± 4	/	/	18.0	15.4 ± 0.3
	Melamine phosphate lignin/rice husk ash		30/5	83 ±3	/	/	42.8	24.3 ± 0.8

Natural fibers

Natural fibers from various sources have been effectively utilized to improve the flame-retardant properties of PHA copolymers, specifically poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB).^187,188 The referenced studies investigated the kenaf fibers that are composed of cellulose, lignin, hemicelluloses, and a small amount of ash. The hydroxyl-rich kenaf fibers exhibited a beneficial effect as a carbonization agent, contributing carbon-rich content that aids in the formation of a char layer. Nonetheless, it is important to note that, natural fibers fall short in some instances. Battegazzore and co-workers¹⁸⁹ conducted a study on poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH abbrevated as PHB on thier study) copolymers that were prepared by melt-blending with hemp hurd (HHC) and alfalfa (AA) fibers. The study found that the addition of both types of natural fibers into the PHBH copolymers resulted in composites with higher viscosity than neat PHBH as illustrated in Figure 6. The increased viscosity was a physical consequence from the added resistance to flow of the filler within the melt. The study argued that higher viscosity was beneficial for flame retardancy as it helped prevent dripping during combustion. This is crucial in preventing the spread of fire, as drips can ignite other materials. In analyzing their data from the cone calorimetry tests, these natural fibers significantly aided the reduction of the HRR. However, their findings also revealed that the presence of these HHC and AA fibers reduced the LOI value and TTI of PHBH. In addition, challenges in completely preventing the spread of flames by the formed char under UL-94 tests were still present. This suggests that the natural fibers made the PHB more flammable, as TTI is decreased, and less oxygen is needed to sustain combustion. This observation contradicts with the benefits of increased viscosity melt as a means for slowing down the diffusion of volatile degradation products and aids in the development of a stable char layer. In addition, it should be noted that when viscosity is too high, it can hinder processability and impede proper mixing and dispersion of fillers within the polymer matrix. Improper mixing and poor dispersion of fillers can potentially lead to inhomogeneous material properties. Therefore, when designing flame-retardant composites, balancing the polymer melt viscosity with flame retardancy is crucial. The goal is to have enough viscosity to improve flame retardancy without compromising processability or overall material performance.

Figure 6.

Complex viscosity (η*) of PHB and PHB-based composites.¹⁸⁹

Nanoparticles

Enhancing the flame retardancy of PHAs through phosphorylation of nano-clay flame retardants is also an emerging area of research.¹⁹¹ Xu and co-workers⁹⁴ proposed an innovative method involving the intercalation of phosphorus-nitrogen-containing hyperbranched macromolecules (HBMs) between bentonite (BNT) layers (HBM-B). The study reported significant improvements in the flame retardancy of PHA, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate). The addition of 15 wt.% HBM-B resulted in an LOI increase of up to 29.6% from 19.8%. The same composite achieved a V-0 rating in the UL-94 vertical burning test. Table 4 shows the results obtained from LOI and UL-94 tests for the composites. Furthermore, the study reported substantial reductions in heat release rate, total hear release, and total smoke production (TSP), along with a significant increase in char residue yield, during cone calorimeter tests (Figure 7). The pHRR of PHA decreased from 981.9 kW/m² to 874.1 kW/m² with the addition of HBM-B. Similarly, the THR decreased from 104.2 MJ/m² to 94.1 MJ/m². While the reductions in pHRR and THR observed with the addition of HBM-B do indicate improvements in flame retardancy, the extent of the decrease is limited. Perhaps, further enhancements could be pursued to maximize flame-retardant performance. For the same composite, the TSP reduced indicating that fewer volatile compounds are released during combustion. It may be beneficial to assess the presence and concentration of toxic gases in the combustion products. This may provide a more comprehensive evaluation of fire safety beyond just the TSP values.

Table 4.

The results of vertical combustion for PHA/HBM-BNT composites⁹⁴

PHA/HBM-BNT (wt.%)	LOI (%)	t₁, t₂ (s)	Dripping	Cotton ignition	Rating
0	19.8 ± 0.3	5, 4	Serious	Yes	V-2
5	27.3 ± 0.4	2, 19	Serious	Yes	V-2
10	28.5 ± 0.2	3, 14	Light	Yes	V-1
15	29.6 ± 0.5	2, 4	Light	Yes	V-0

Figure 7.

(a) HRR, (b) THR, (c) TPS, and (d) Residual mass of PHA and PHA/15 wt.% HBM-B obtained from cone calorimetry.⁹⁴

Wu and co-workers¹⁹⁰ investigated the synergistic effects of melamine phosphate modified lignin (MAP-lignin) and rice husk ash (RHA) on the flame-retardant properties of P(3HB-co-4HB). Rice husk contains mainly amorphous silica ash after burning.¹⁹² Using RHA as a flame retardant is considered environmentally friendly, as it utilizes an agricultural waste product. The study found that the addition of 30 wt.% MAP-lignin and 5 wt.% RHA significantly improved the pHRR and THR of P(3HB-co-4HB) in cone calorimeter tests. In addition, the study included morphological and microstructural analysis of char residues obtained from the cone calorimeter tests using scanning electron microscopy with energy dispersive X-ray (SEM-EDX) analysis. The micrographs from this analysis are shown in Figure 8. The deduction was that in the presence of phosphorylated lignin, the composite exhibited a more compact and coherent structure. This morphology indicates a more effective barrier against heat and mass transfer. This was observed with an increase in carbon content up to 41 wt.%, attributed to higher char formation, and the presence of 9 wt.% phosphorus.

Figure 8.

SEM images of char residues after cone calorimeter measurements: (a) P(3HB-co-4HB)/RHA, (b) P(3HB-co-4HB)/10 wt.% alkali lignin, and (c) P(3HB-co-4HB)/10 wt.% MAP lignin.¹⁹⁰

Bio-based flame retardants for starch-based plastics

Starch is a natural polymer abundant in various plants and is primarily extracted from corn, potatoes, wheat, and rice.^193,194 Starch is a polysaccharide composed of glucose units.¹⁹⁵ It consists of two main components: amylose and amylopectin.¹⁹⁶ Amylose is a linear polymer of glucose units linked by α-1, 4-glycosidic bonds. Amylose typically makes up 20%–30% of starch and is responsible for the crystalline structure of starch granules.^197,198 Amylopectin is a branched polymer with α-1, 4-glycosidic bonds and α-1, 6-glycosidic linkages at the branch points. Amylopectin constitutes 70%–80% of starch and contributes to the amorphous regions of starch granules.^193,197 Starch occurs naturally in the form of granules, which vary in size and shape depending on the plant source.¹⁹⁹ These granules are semi-crystalline, with alternating crystalline and amorphous regions.

Thermoplastic starch (TPS) is a biodegradable material derived from starch.²⁰⁰ To transform starch into a thermoplastic material, it is mixed with plasticizers, typically water, glycerol, or sorbitol.²⁰¹ This process disrupts the hydrogen bonds in the starch granules, allowing the material to flow when heated and be molded into different shapes. TPS is produced through thermal processing methods such as extrusion or injection molding. During this process, the plasticizers and heat facilitate the gelatinization and melting of starch, creating a homogeneous, flexible material.²⁰² Other additives like compatibilizers, fillers, and reinforcing agents can be incorporated to enhance the mechanical properties and processability of TPS.^203–205 TPS offers a sustainable and biodegradable alternative to traditional plastics, leveraging the abundant and renewable nature of starch. Table 5 summarizes the flame-retardant systems that have been applied to starch. The table provides quantitative data on the flame performance metrics to illustrate the enhanced effectiveness. The data include key parameters such as TTI, UL-94 rating, LOI, pHRR, and char yield.

Table 5.

Dual function of thermoplastic starch in flame retardant systems and their effectiveness

Biopolymers matrix	Flame retardant	Method of application	FR loading (Wt.%)	TTI (s)	UL-94	LOI (s)	pHRR (%)	Char residue (%)	Ref.
Thermoplastic starch (TPS)	Choline phytate	Bulk (melt-blending)	30	/	V-0	43.7	/	/	Wang et al.²⁰⁶
PLA	/	Bulk (melt-blending)	/	80.2 ± 0.9	/	/	/	0.0	Maqsood and Seide²⁰⁷
	Oxidized starch/APP		1/10	65.5 ± 2.6	/	/	39.0	15.1 ± 0.5
	Oxidized starch/APP		7/10	50.4 ± 3.4	/	/	52.0	33.5 ± 0.9
TPS	/	Reactive—Bulk (melt-blending)	/	/	/	19	/	12.9	Liu et al.²⁰⁸
	Phytic acid		10	/	/	34	65.6	20.2	Liu et al.²⁰⁸

Phytic acid and nanoparticles

TPS is a plant-based biopolymer that is highly flammable and hygroscopic. In the development of fully biodegradable starch-based materials, the incorporation of flame retardants must be carefully managed to ensure both effectiveness and biodegradability. The incorporation of choline phytate (CPA) into TPS significantly enhanced the flame-retardant properties of the composite.²⁰⁶ This was evidenced by improved UL-94 ratings, higher limiting oxygen indices, and reduced heat release rates and total heat released in cone calorimeter tests.

Intumescent formulations consisting of oxidized starch (OS) as a carbonization agent, APP, and PLA were successfully prepared using a twin-screw extruder.²⁰⁷ These formulations exhibited enhanced flame retardancy evidenced by improved heat release rate, THR, TTI, and residual mass percentage. The mechanism behind these improvements was attributed to the effective char formation promoted by CPA and APP ring combustion, respectively.

The research by Liu et al.²⁰⁸ also highlights a promising approach to enhancing the flame retardancy of TPS by leveraging the synergistic effects between PA and HNTs. This combination not only improves fire resistance but also maintains the material’s eco-friendly attributes, making it a valuable strategy for developing safer, more sustainable polymer composites.

Discussion of results

The development of effective flame retardants for biopolymers is crucial for expanding their applications in areas requiring fire safety. The global flame-retardant market was valued at USD 8.63 billion in 2022, with a projected compound annual growth rate (CAGR) of 7.1% from 2023 to 2030. APP and metallic hydroxides, for example, MH and ATH, are among the most effective conventional flame retardants.^209–211 Furthermore, these flame retardants are also cost-effective compared to other halogen-free systems. However, despite their advantages, APP, MH, and ATH show limited efficiency as flame retardants when used alone in some biopolymers, such as PLA and PBS.^212–215 Moreover, metallic hydroxides are known to require higher loading amounts in polymers. This ensures compromised mechanical property and processability. Accordingly, when used in PLA, phosphorus-containing flame retardants, including APP, are often part of intumescent formulations.²¹⁶ Typically, 20%–30% of such flame retardants are required to raise PLA’s flame retardancy from a non-classifiable state to a V-0 rating, with an increase in LOI to 38.²¹⁷ Synergistic combinations of APP with inorganic particles, such as melamine (MEL), MMT, aluminum hypophosphite (AHP), or multi-wall carbon nanotubes (MWCNT), have also demonstrated significant improvements in flame retardancy.^218–220

While considerable progress has been made, ongoing research is focused on enhancing the flame-retardant efficacy and environmental safety. This review consolidates the current state of bio-based flame-retardant strategies for biopolymers, highlighting their mechanisms, effectiveness, and challenges. This shift toward bio-based solutions not only supports environmental sustainability but also addresses the growing need for greener alternatives in flame retardancy. Indeed, improved fire performance can be achieved using bio-based flame retardants, which also offer the benefit of reduced environmental impact. In most cases, bio-based additives, such as lignin, cellulose, PA, CS, TA, among others, have a charring ability when burnt or promote char formation. The char formed acts as a protective barrier, reducing heat transfer and slowing down the degradation of the underlying material, thereby enhancing flame retardancy. Due to this property, these typical additives have potential applications in the development of sustainable flame-retardant biopolymer composites. However, important concerns remain that warrant further investigation.

Similar to conventional methods, one major concern is the low effectiveness of bio-based flame-retardant fillers when used alone. Their effectiveness is significantly enhanced through modifications with phosphorus-based or/and nitrogen-based compounds, or by combining them with nanomaterials.^{86,107,108,174,179,190} In addition, advanced preparatory methods like a combination of grafting and melt-blending or LbL assembly are used to further improve their performance.^105,109,208 Figure 9(a) and (b) shows data reproduced from the referenced studies to highlight these improvements. However, some shortcomings have also been reported due to the nature of the filler or its weight percentage composition.^110,111,174 These shortcomings are often characterized by a decreased TTI (Figure 9(c) and (d)).^{174,175,179,207} Compromised TTI is an alarming drawback in flame-retardant applications. When the TTI is compromised, it means the material ignites more quickly than desired, reducing its overall effectiveness. This could happen due to the specific composition of the flame retardant, the nature of the filler used, or its interaction with the polymer matrix. Another area of concern is the lack of complementary results in mainstream research studies. It is often believed that some studies focus only on cone calorimetry, while others emphasize LOI or TGA, or UL-94 V-rating, leading to an incomplete understanding of overall fire performance of the composite.^103,176,207 Addressing these gaps and concerns through comprehensive, multi-faceted research will be critical in advancing the field of bio-based flame retardants for biopolymers.

Figure 9.

Effectiveness of (a) lignin & (b) chitosan on PLA, and (c) phytic acid & (d) lignin on PBS flame-retardant fillers before and after modification/combination with modifiers. Data reproduced with permission from Costes et al., Chen et al., and Kong et al.^{107,108,174,179}

Bio-based flame retardants may be somewhat more expensive than conventional flame retardants. This price disparity can be attributed to factors common to many environmentally friendly alternatives, where healthier and more sustainable solutions often come with higher costs. However, the use of renewable and greener processes in the production of bio-based flame retardants can help salvage some of the cost disadvantages. These eco-friendly methods not only promote sustainability but also reduce reliance on non-renewable resources, which can lead to long-term cost benefits. In addition, much like conventional flame-retardant systems, bio-based alternatives often require further modification or combination with synergistic additives to enhance their effectiveness. These necessary modifications add to the overall cost, although they are essential for achieving the desired performance in fire resistance. However, as green chemistry and sustainable manufacturing processes continue to advance, these costs could decrease over time, making bio-based flame retardants a more competitive option in both environmental and economic terms.

Conclusion and future directions

The innovative approach to enhance the effectiveness of bio-based flame retardants, particularly through phosphorylation, shows great promise. In addition, nitrogen-containing and nano-structured compounds can act synergistically with carbon- and phosphorus-based flame retardants to further improve fire resistance. As a result, renewable phosphorus-containing and nitrogen-containing additives such as DNA, PA, TA, casein, and hydrophobins are potential candidates for biopolymers. This broad applicability highlights the potential of bio-based flame retardants to improve the fire safety of various biopolymeric materials while maintaining environmental sustainability. Therefore, future research should focus on conducting studies that simultaneously assess multiple aspects of flame retardancy, including mechanical properties and comprehensive fire performance metrics. This will provide a more complete understanding of how different flame retardants perform in various scenarios.

The effect of bio-based flame retardant on the biodegradability of biopolymers is another key area to study. Flame-retardant additives could potentially alter the chemical structure and physical properties of biopolymers, which might affect their ability to biodegrade under natural conditions. Furthermore, in some cases, biodegradability is not a desired property, particularly for applications requiring durability. Hence, it is essential to develop flame retardants that maintain or enhance biodegradability without compromising the fire resistance of the material. Developing flame retardants that are compatible with biopolymers and effective at low loadings may be a solution. It is important to emphasize that the effect of flame-retardant additives on the biodegradation of biopolymers is yet to be thoroughly studied. While flame-retardant additives are crucial for enhancing the fire resistance of biopolymers, their impact on the material’s biodegradability remains unexplored. For this to be done, further empirical studies and advanced characterization techniques are recommended.

Footnotes

Author contributions

Thobile L Mhlabeni: Conceptualization,Investigation,Methodology,Roles/writing—original draft,Formal analysis,Writing—review and editing,and Validation. Endazenaw B Chemere: Writing—review and editing,Validation. Tamba Jamiru: Writing—review and editing,Validation,Funding acquisition. Washington Mhike: Conceptualization,Writing—review and editing,Validation,Supervision,and Funding acquisition.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: Open access funding provided by Tshwane University of Technology. The authors received no direct funding for this research.

ORCID iDs

Thobile L Mhlabeni

Washington Mhike

Author biographies

Thobile L Mhlabeni is a Postdoctoral Researcher at Tshwane University of Technology in Pretoria,South Africa. She earned her Ph.D. in Chemical Engineering from the University of Pretoria in 2023. Her current research centers on the development and application of advanced biofreindly nanostructured and polymeric materials.

Endazenaw B Chemere is an Assistant Professor of Chemistry. He receveid two Ph.D degrees from National Taiwan University of Science and Technology (NTUST) and Ming Chi University of Technology (MCUT) in 2021 and 2022,respectively. Currently,he is a postdoctoral research fellow at Tshwane University of Technology in Pretoria,South Africa.

Tamba Jamiru is an Associate Professor in Mechanical Engineering at Tshwane University of Technology. His research focuses on mechanical nanomaterials,advanced composites and phase transfomation,contributing to advancement in Materials science and Engineering.

Washington Mhike is a faculty member in Chemical,Metallurgical and Materials Engineering at the Tshwane Universty of Technology. He earned his Ph.D. in Chemical Engineering from the University of Pretoria in 2016. His research addresses scocietal challenges,focusing on malaria prevention,polymeric applications in water treatment,compostable medical decices and eco-friendly flame retardants.

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